There are a number of methods for providing direct current (DC) with a superimposed alternating current (AC). In each of these methods it is necessary to provide isolation between the DC and AC sources. A typical method of such decoupling is illustrated by a diagram in FIG. 1. An inductance (3) blocks the flow of alternating current into a DC source (1) while a capacitor (4) prevents an AC source (3) from short circuiting the DC power supply. A power supply of this type which provides an AC current superimposed on DC current to a load (5), however, becomes unwidely in applications where large currents (of the order of thousands of amperes) are required as in the case of some electrochemical installations. In such cases the values of inductance (3) and capacitance (4) become quite largely and prohibitively expensive.
A method that provides a partial solution to these problems is illustrated by a diagram in FIG. 2. Alternating current is provided by a transformer (11) with a center tap (12). A DC power source (13) is connected between the center tap (12) on the transformer and the common point (14) of two loads (15 and 16). If the loads are identical, then AC voltage across the DC supply (13) is zero and no inductance is required to prevent the alternating current from flowing through the DC source. Also no capacitor is required in this circuit.
The main disadvantage of this approach is the requirement that the two loads be identical. The need for balanced loads creates a number of difficulties in practical applications and has the effect of increasing the cost of industrial processes which require the use of such supplies.